High power fiber lasers and amplifiers are effective converters of low brightness radiation (optical pump light) from diode-lasers to high brightness single-mode laser radiation. Such lasers and amplifiers have an “all-fiber” construction, and exhibit excellent stability and excellent beam quality. High power fiber lasers and amplifiers can be a viable alternative to “bulk” solid-state lasers for scientific and industrial applications.
A fiber laser has a higher overall efficiency than a bulk laser, for example, about twice the efficiency. In continuous wave (CW) operation, a fiber laser can have an optical-to-optical efficiency approaching 80%. Such a high efficiency is achieved by keeping optical pump light and signal light in a fiber core with no diffraction loss and no reflection from bulk intracavity elements. In pulsed operation, however, the efficiency of fiber lasers drops significantly, for example to between about 10% and −30% of the CW efficiency. A reason for this is as follows. In order to modulate (pulse) the light, bulk acousto-optical or electro-optical modulators are used. This requires that light is coupled out of a fiber in order to be modulated and then coupled back into another fiber. Coupling light out of a fiber and back into a fiber results in 20%–40% extra loss in a fiber laser cavity with bulk modulators.
For pulsed operation, a master oscillator power amplifier (MOPA) system is preferred. In such an arrangement, a relatively low power laser provides laser radiation pulses having desired characteristics. These pulses are then amplified to high power in an efficient fiber amplifier. The efficiency of the fiber amplifier determines the overall efficiency of the MOPA system.
One common MOPA system comprises a master oscillator and a multistage amplifier. The master oscillator can be a solid-state laser, a fiber laser, or a semiconductor laser that provides light pulses having required optical parameters. These parameters include spectral width, pulse repetition rate, and pulse length. In a pulsed (modulated) diode-laser, pulse length and pulse repetition rate can be independently controlled. In a solid-state laser, and in a fiber laser, pulse length changes with repetition rate.
In order to amplify low power pulses, for example, pulses with less than one-watt (<1.0 W) of power, to pulses having a power of 1.0 Kilowatts (KW) or more, multiple amplification stages are necessary. A rare-earth-ion-doped silica-fiber amplifier can provide high gain (up to about 30 dB) for a small signal. Such strong gain causes a self-excitation of the amplifier due to back-reflection from fiber ends. This is termed amplified spontaneous emission (ASE). Strong gain and back reflection can also give rise to cross-talk between amplifier stages in a multi-stage amplifier. ASE from one amplifier stage can be amplified in a second stage. This self excitation takes part of the stored energy (resulting from optical pumping) of the second stage, which would otherwise be available for signal amplification. This can lead to instability in a fiber MOPA system with multistage amplification.
Suppression of back reflection and isolation between amplifier stages is necessary to combat instability in a MOPA system with multistage amplification. Typically, rare-earth ions in silica have a broad spectrum of spontaneous (fluorescence) emission. By way of example, the fluorescence spectrum of an ytterbium (Yb) doped silica fiber has a fluorescence band extending from about 1020 nanometers (nm) to about 1180 nm, and a strong separate fluorescence peak at 976 nm. Because of this, broadband isolation is required.
In prior-art MOPA systems, bulk optical elements such as isolators, spectral filters, acousto-optic modulators and the like have been used between amplifier stages. Such bulk optical elements are at least partially effective, but have certain shortcomings. By way of example, an isolator provides only unidirectional isolation and an acousto-optical modulator introduces high insertion losses in a system and is only effective when in a closed condition. A narrow passband spectral filter can be effective in reducing the spectral width of ASE to about 1 nanometer. It is difficult, however, to make such a narrow-band spectral filter with strong (>15 dB) rejection of wavelengths in a broadband ASE spectrum corresponding, for example, to Yb ions.
Any MOPA system wherein isolation elements add loss may require more amplification stages than a system without such amplification stages. These additional stages add cost to a MOPA system in addition to the cost of the lossy, bulk isolation elements that create the need for these added stages. Accordingly, there is a need for a method and apparatus for providing more effective ASE and cross-talk suppression in a multistage MOPA system. There is also a need for such a method and apparatus that does not require bulk optical elements and that can provide, with less amplification stages, at least the same overall gain as a prior-art MOPA system.